The present invention is related to a force transducer capable of measuring a force and/or moment exerted upon it according to one or several directions. Use is made of optical fibres which are strained when a force is acting on the transducer and which are configured to provide a change in a detectable optical property responsive to the strain. In particular, the force transducer according to the invention is configured for use in minimally invasive surgery, such as for measuring forces acting at the distal end of a catheter.
Nowadays, the trend in surgery is to operate in a minimally invasive way inside the patient in order to reduce trauma and potential complications. In gastroenterology, the use of endoscope allows the surgeon to work with a set of medical tools (knives, needles, etc.) inside the digestive system.
When using an endoscope, the manipulation of the catheter driving any medical tool is subject to parasitic forces, such as the friction between the catheter and the endoscope, the elastic force due to the change of curvature of the catheter and the body of the medical tool. Consequently, the force felt by the surgeon through the catheter is not the force exerted by the tool on the patient body. There is therefore need of a force measurement system in order to feed back the force exerted by the medical tool on the patient body. This requires the force to be measured as distally as possible on the tool.
To this end, force transducers are known from WO 2009/007857, which comprise two base portions spaced apart by one or more standoff members and a trio of strain sensing elements. The strain sensing elements can include optical fibres potted in the base portions and defining interferometric gaps between a transmitting part of optical fibre and a receiving element, which may act as extrinsic Fabry-Perot interferometers (optical fibres are not stressed).
The standoff member can assume different forms and serves the purpose of (weak) link between the base portions, so that when a force is acting on one of the base portions, the interferometric gap is altered and a measurement can be performed. The standoff member has a coefficient of thermal expansion similar to that of the strain sensing elements to render the measurement insensitive to bulk temperature changes, and can be made of a same material. However, it may be difficult and/or expensive to provide a standoff member of same or similar material as the optical fibre and in a desired shape.
WO 2010/079418 describes, relating to FIGS. 17-21, another force transducer of the above type, which comprises a structural member defined by segments linked to one another by flexures defining a serial arrangement of gaps in the structural member. A plurality of optical fibres are attached to the segments and comprise Fabry-Perot or Bragg grating strain sensors to measure the change of distance between the segments. When the structural member is deformed by a force acting on it, the optical fibres are strained, which is sensed by the strain sensors shifting the wavelength of the reflected light.
Such force transducers have the advantage that they can be made very small, with representative dimensions in the order of magnitude of a few millimeters, so that they can be incorporated at the distal end of a catheter and be inserted without any problem through the lumen of an endoscope.
A disadvantage however, is that the optical fibres have different lever arms relative to the structural member for each optical fibre, which complicates force measurement. The transducer moreover has a limited size below which the structural member does not present an appropriate stiffness to measure usual force in minimally invasive surgery.
Another disadvantage of the above force transducers, is that, considering the small dimensions, they are relatively difficult to manufacture, and consequently, their manufacture is costly.